I would like to make a clever analogy that explains why HBP disappoints me, like, perhaps, "HBP is The Magician's Nephew of the series." Except it's not true. I liked The Magician's Nephew quite a bit, even if it introduces us to characters we don't know--or that we think we don't know, until later--because it's an active tale, not an expository one. It doesn't give me the sense of "You couldn't have explained this in two pages? You couldn't have explained this in two pages four books ago?" that HBP does. Pensieve exposition is still exposition.
The trailer really brings home how little--how little of importance, anyway--Harry has to do in this book.